Many industrial processes result in the emission of small hazardous particles into the atmosphere. For example, toxic airborne byproducts of coal combustion tend to concentrate in the fine particulate fraction of emissions due to the heavy metals and heavy organic material in the coal. Many of the trace metals, such as arsenic, cadmium, nickel, selenium and their compounds, volatise at the high combustion temperatures and either nucleate homogeneously or condense predominantly on the fine fly ash particles as the flue gases cool. The same is true of some of the hazardous organic air pollutants.
The toxic particles which are formed by homogenous nucleation are very fine sub-micron particles. As these fine particles are able to enter the human respiratory system, they pose a significant danger to public health. The identified combination of toxicity and ease of respiration has prompted governments around the world to enact legislation for more stringent control of emission of particles less than ten microns in diameter (PM10), and particularly particles less than 2.5 microns (PM2.5). Government regulations controlling particulate emissions are likely to become more stringent in the future, especially for fine particles in the micron and sub-micron size range, as the hazardous effects of such particulate emissions become more widely known.
Smaller particles in atmospheric emissions are also predominantly responsible for the adverse visual effects of air pollution. For example, in coal burning installations, stack opacity is largely determined by the fine particulate fraction of the fly ash because the light extinction coefficient peaks near the wavelength of light which is between 0.1 and 1 microns.
The importance of fine particulate control can be appreciated by consideration of the number of pollutant particles in an emission rather than the pollutant mass. In fly ash from a typical coal combustion process, pollutant particles less than 2 microns in size may amount to only 7% of the total pollutant mass, yet account for 97% of the total number of particles. A process which removes all the particles greater than 2 microns may seem efficient on the basis that it removes 93% of the pollutant mass, yet 97% of the particles remain, including the more respirable toxic particles.
Various methods have been used to remove dust and other pollutant particles from air streams. Although these methods are generally suitable for removing larger particles from air streams, they are usually much less effective in filtering out smaller particles, particularly PM2.5 particles.
It is known to use particle agglomeration techniques to combine smaller particles into larger particles, which can then be removed more easily or effectively. Known agglomeration techniques include: (i) injection of chemicals into air streams to increase agglomeration of fine particles, (ii) use of laminar flow precipitators to promote surface agglomeration of fine particles, (iii) acoustic agitation of dust particles suspended in a gas to increase impingement and hence agglomeration rates, (iv) AC or DC electric field agitation of charged dust particles suspended in a gas to increase mixing and hence agglomeration, and (v) bipolar charging of particles in a gas stream for electrostatic attraction.
An example of a known surface agglomeration technique can be found in U.S. Pat. No. 5,707,428, while an example of the AC field agitation method can be found in European patent application no. 0009857.
These techniques are usually costly to implement in large scale installations, and the chemical injection method raises other health concerns. Further, the known techniques are not particularly efficient in relation to fine dust particles.
The most common agglomeration technology is surface agglomeration. In surface agglomeration techniques, particles must be brought into contact with a collecting surface or body to be removed from the gas stream. Large particles, greater than about 10 microns in diameter, are captured relatively easily by inertial mechanisms such as impaction, interception and centrifugal forces. In electrostatic precipitators, large particles are more easily collected as they experience greater electrical forces due to their capacity for greater charge.
However, as particle size decreases, the mass of the particle decreases in proportion to the cube of the diameter, and inertial forces are less effective in bringing the particles to a collecting surface. These small particles also hold less charge, and therefore experience smaller electrostatic forces. For particles less than 0.1 micron, diffusion is usually the main mechanism for particle transport, charging and capture. For particles between 0.1 and 2 microns however, neither diffusive, electrostatic nor inertial mechanisms are very strong, and known devices which utilise these mechanisms usually exhibit minimum collection efficiency in this size range.
The effectiveness of diffusive capture may be increased by providing greater surface area and/or more time for diffusion to occur, but a significant increase in equipment size is required. Greater inertial forces can be obtained by increasing the relative velocity of the particle to the collecting surface, but at the expense of greater pressure drop and power input to the collecting device, which results in greatly increased costs. Hence, economic considerations have limited these approaches.
Other dust collection devices that have been used for fine particle emission control include wet electrostatic precipitators and scrubbers. These normally require large and expensive installations, and give rise to the problem of disposal of contaminated wastewater. Fabric filters have also been used as dust collectors, but they tend not to be efficient collectors of fine particles as the small and generally smooth particles tend to bleed through the fabrics used in such filters.
It is an object of this invention to provide an improved method and apparatus for particle agglomeration.